Do not map read-only data memory sections with EXECUTE flags.

Do not map read-only data memory sections with EXECUTE flags.

The code in SectionMemoryManager.cpp unnecessarily maps
read-only data sections with the READ+EXECUTE flags. This is
undesirable from a security stand-point.

Moreover, on the Fuchsia platform, which is now very strict
about mapping pages with the EXECUTE permission, this simply
fails, because the section's pages were initially allocated
with only the READ+WRITE flags.

A more detailed description of the issue can be found in this
public SwiftShader bug:

  https://issuetracker.google.com/issues/154586551

This patch just restrict the mapping to the READ flag for ROData
sections. Code sections are still mapped with READ+EXECUTE as
expected.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D78574